Data warehouse is a system/architecture which have historic,stable,sizeable,non-volatile data. Usually datawarehouse is used in corporate world to keep track of their historic data including the firm's progress,turn-over,employee/clients's information etc.
DataWarehouse is a collection of Data Marts.eg: if there are 4 departments in a company then there are 4 data marts respectively.(Customer,Product,Employee,Vendors etc).
The collection of these data marts forms a Data Warehouse.
And to extract or perform any operation on this datawarehouse various DataWarehousing tools are used such as Informatica(Extraction Transition Loading "ETL" tool),
Cognos & Business Objects (BO)-OLAP tools.
I hope this has given you some general idea about Data Warehouse.